I have a 98 DeVille Northstar that is driving me nuts. It sometimes overheats, usually after driving for about an hour. It has had two new thermostats and a new radiator. Been to three different shops and no one can fix it. Water pump has not been replaced. Sometimes when it gets to about 245 degrees I will hear a squeal and the temp will then fall. Sitting at idle it will heat up to 225, the fans will come on and it will cool back to 210.

One mechanic says the probem is a head gasket, which would cost more than the car is worth. Is it possible it is the idler on the belt for the water pump? Or can it be a bad head gasket?

I would have to guess that it is your head gaskets. I am a loyal GM fan and I have two GM products that are '98 models. Both of them had to have head gaskets put on them last year. My truck is a deisel so it was very obvious, the high compression ratio would not let water stay in the system. My wifes van is a Venture and it would lose a small amount of coolant over a month or two. That went on for better than a year. Eventually, it began to get hot and lose a fair amount of coolant. The head gaskets are a graphite compound with metal ringed edges. For some reason, GM decided not to ring the holes where the coolant jacket passes from block to head. If the gasket is not compressed exactly the right amount of torque, the metal rings around the combustion chamber will crack, eventually causing the graphite to push into the water jacket.

This may not be the case for you but I would bet my rent on it. I had two in the same year, both '98 models.

I can say that I don't have any personal experience with NorthStar engines, and I am told they are a different breed of animal, so you definitely want to keep asking around. Not much help.

Northstars are prone to overheating, most of the time it is head gaskets.
Northstarts engines pull the head bolt threads so the head actually becomes loose. You need to remove heads and insert timeserts whcih are basically new threads for the head bolts. I've doen this once to a northstar, I have diagnosed many and most people will trade off the car because of the expense.